This invention relates to a process for producing alumina. In particular, the invention relates to a process for extracting alumina from bauxite.
Aluminium is produced, almost exclusively, from the alumina ore bauxite by a combination of the Bayer and Hall-Heroult processes. In the Bayer process, bauxite is typically crushed, washed, dried and ground before being treated with aqueous sodium hydroxide at an elevated temperature. The sodium hydroxide dissolves the alumina to form a solution from which insoluble impurities (xe2x80x9cred mudsxe2x80x9d) are removed. Alumina trihydrate (Al2O3.3H2O or Al(OH)3) as gibbsite is precipitated from the solution and is calcined to produce alumina (Al2O3) for electrolytic reduction to aluminium in the Hall-Heroult process. An outline of the Bayer process is given in Chemistry and Industry, Jul. 18, 1988, p.445-451.
The precipitation of the trihydrate from the solution (xe2x80x9cthe Bayer process liquorxe2x80x9d) involves seeding the solution with crystals of the trihydrate (in the form of gibbsite) at about 80xc2x0 C. and maintaining the temperature at this level for a significant period of time whilst crystallisation takes place. If the trihydrate is precipitated more rapidly, for example at lower temperatures, it is found to be less pure and the impurities which it contains, such as silicates, sulphate, oxalate, carbonate, sodium and organic compounds, make it unsuitable for electrolysis because of the adverse effects which these impurities have on the electrolytic cell and on the resulting aluminium.
An essential feature of the Bayer process is that the liquor stream is recycled; as a consequence, there can be a build up of contaminants, impurities and additives with time. In Bayer liquors containing relatively high levels of oxalate as impurity, this problem is remedied to some extent by crystallising sodium oxalate from the liquor.
The contaminants in the Bayer liquor may be conveniently considered under three separate headings: organic impurities, inorganic impurities and additives.
Organic materials are introduced into the Bayer process by almost all bauxites. Whilst initially much of this is in the form of humic materials, with time in the liquor, degradation occurs and a vast number of organic compounds is generated, with a wide range of molecular weights and chemical structures and functionalities. The largest concentration of any single organic compound is the oxalate anion (C2O42xe2x88x92), the penultimate degradation product of the humates present in the liquor prior to the formation of CO2/CO32xe2x88x92. Not all of the organic compounds have equal effects on the Bayer process. Many of the organic compounds strongly absorb visible light, turning an otherwise colourless liquor various shades from pale straw to virtually black. The most serious problem associated with many of the organic compounds is the undesirable effects they have on the precipitation of gibbsite and sodium oxalate.
Certain compounds are known to be very strong poisons of gibbsite precipitation. For example, organic molecules containing specific hydroxyl (xe2x80x94OH) and carboxyl (xe2x80x94COOH) groups can dramatically affect gibbsite precipitation. Negative effects of these compounds can include reduction in precipitation rate, precipitation yield, product purity (both colour, and also increased sodium content, which remains in the alumina produced by calcining, and causes problems in subsequent aluminium production), morphology and strength (resistance to attrition) and an increase in the relative amount of smaller particles (fines) in the product.
In all Bayer plants where there is an increasing concentration of oxalate in the liquor stream with time, the sodium oxalate must be removed, because above certain concentrations the liquor becomes unstable with respect to the solubility of sodium oxalate, and the latter can precipitate out spontaneously, causing various problems in plant operation, including flotation and solids overflow, generation of undesired fine particles of gibbsite and contamination of the gibbsite product by co-crystallization with sodium oxalate. The controlled removal of sodium oxalate is usually achieved by controlled precipitation, either in a separate side stream or by deliberate co-crystallization with gibbsite, in a controlled manner, in the main process stream. Alternative removal processes include xe2x80x9cliquor burningxe2x80x9d and adsorption onto an inert solid phase e.g., activated carbon. Some of the organic compounds in the Bayer liquor, apart from oxalate, can affect the precipitation of sodium oxalate by raising its apparent solubility (and hence lowering the driving force to make it crystallize), thereby reducing yield, as well as precipitation rate, by poisoning sodium oxalate crystal growth sites. The compounds which have a negative effect on the crystallization of sodium oxalate are referred to by the generic term oxalate seed poisons.
Various inorganic anions are introduced into the Bayer liquor during plant operation, some of which build up in concentration, through digestion of the bauxite, from naturally occurring impurities in the bauxite and/or reagents and water, and, in the case of carbonate (CO32xe2x88x92), also through reaction of atmospheric carbon dioxide with the caustic soda solution.
Inorganic anions derived from impurities in bauxite include oxoanions of metals (e.g., oxoanions of transition metals) such as vanadate (VO43xe2x88x92), ferrate (FeO42xe2x88x92) and silicate (SiO44 ). Other inorganic anions in the liquor include sulphate (SO42xe2x88x92) and phosphate (PO43xe2x88x92), for example.
In the case of some Bayer plants, using certain bauxites as feedstocks, the concentration of sulphate in the liquor can build up to unacceptable levels. The problems caused by the presence of sulphate in the liquor stream include raising the ionic strength of the liquor whilst reducing the effective level of free caustic, and potential problems of scale formation (e.g., by gypsum, CaSO4.2H2O) through reaction with calcium added during causticisation.
During the digestion stage of the Bayer process, soluble silica is taken up into solution in the Bayer liquor, and is subsequently precipitated as de-silication product (DSP), which partially or wholly consists of one or more sodium alumino-silicates. Whilst this precipitation step effectively removes the silicon from solution, it does so at the expense of removing sodium and aluminium from useful production.
The conversion of sodium hydroxide in the Bayer liquor to sodium carbonate greatly reduces the effectiveness and productivity of plant operation, and so is currently treated in many refineries by the process of causticisation, in which quicklime (calcium oxide, CaO) and/or slaked lime (calcium hydroxide, Ca(OH)2) is added to the liquor. In practice, not all of the lime is converted to calcium carbonate, and some reacts to form other compounds, including ones which use up aluminium. The large quantities of lime used, combined with the inefficiencies usually associated with its use, leads to causticisation being a large expense in the operation of a Bayer plant, as well as there being associated negative environmental impacts.
Various chemicals are added to the Bayer liquor at different parts of the process to achieve a range of improvements; for example, crystal growth modifiers and flocculants. It is sometimes advantageous to be able to remove some of these chemicals at certain stages of the process, and yet not remove others.
In view of the huge volume of bauxite which is processed around the world, even small improvements in the process, such as a reduction in the time taken for precipitation or a reduction in the temperature which needs to be maintained, can lead to vast cost savings.
Layered double hydroxides are a class of compounds which comprise two metal ions and have a layered structure. A brief review of layered double hydroxides is provided in Chemistry in Britain, September 1997, p. 59 to 62. The hydrotalcites, perhaps the most well-known of the layered double hydroxides, have been studied for many years. Hydrotalcite clays useful as catalysts for hydrocarbon conversion are disclosed in U.S. Pat. No. 5,354,932. It is known that layered double hydroxides can intercalate certain inorganic anionic species, such as carbonate, within the layers in their structure.
The structure of the layered hydroxides [LiAl2(OH)6]X, where X is Cl, Br or NO3, and their monohydrates, has been described by Besserguenev et al., in Chem. Mater, 1997, no. 9, p. 241-247. The hydroxides can be produced by the reaction of gibbsite [xcex3-Al(OH)3] or other forms of Al(OH)3, such as bayerite, nordstrandite or doyleite, with lithium salts of formula LiX. The materials can also be formed in other ways, such as by direct precipitation (see, for example, Serna et al, Clays and Clay Minerals, (1997), 25,384). The structure of the LiAl2(OH)6+ layers in the compounds is unusual for a layered double hydroxide since it is based on an ordered arrangement of metal cations. The preparation of these compounds is also described in U.S. Pat. Nos. 4,348,295 and 4,348,297. A few other layered double hydroxides having cation ordering are known, such as the compounds containing layers of formula [Ca2Al(OH)6]+.
Perrotta et al, Light Metals, 1996, pages 17 to 28 and Light Metals, 1995, page 77 describe layered double hydroxide formation in Bayer liquor and its promotional effect on oxalate precipitation. The layered double hydroxide is formed only in situ by the addition of calcium or magnesium hydroxide to the Bayer liquor and can reduce its oxalate content. Also, the same in situ layered double hydroxide formation is described as enhancing oxalate precipitation from spent liquor. Since the layered double hydroxide is formed in situ in the Bayer liquor, with the dissolved alumina in the liquor providing a component of the reaction in which the layered double hydroxide is formed, the liquor is not itself treated with a layered double hydroxide. The chemistry behind the process is not described in detail by Perrotta et al but there are indications that intercalation plays at most a minor role. A further application of the same concept is described in Perotta et al, Light Metals, 1997, 37 which involves the treatment of Bayer lake water. The technology taught by Perrotta et al requires the use of relatively large amounts of lime or magnesia which, generally, is not recycled and leads to the removal of aluminate ions from the liquor (to form the layered double hydroxide) which is clearly undesirable. Also, it has the disadvantage of potentially introducing unwanted substances into the Bayer liquor; it is known, for example, that calcium ions have an adverse effect on the precipitation of gibbsite (see Cornell et al, Fourth International Alumina Quality Workshop, Jun. 2-7 1996, pages 97-). Furthermore, calcium leads to the formation of CaSO4.2H2O and other scale-forming compounds.
WO 97/03924 describes a method for purifying sodium aluminate solutions containing sodium oxalate (e.g., Bayer process liquors) in which the sodium oxalate is precipitated using tricalcium aluminate hexahydrate. The tricalcium aluminate hexahydrate acts as an initiator for precipitation and is not described as having any effect on the other impurities in the liquor.
The present invention aims to alleviate or solve the problems associated with the presence of impurities in Bayer process liquors. The solution to the problem involves the use of layered double hydroxides.
Accordingly, the present invention provides a process for producing alumina from bauxite which comprises treating bauxite with an alkali to form a mixture comprising a solution of aluminium-containing ions and precipitating alumina trihydrate from the solution, characterised in that the process comprises treating the solution before or after the precipitation step with a layered double hydroxide in order to remove impurities from the solution by intercalation into the layers in the double hydroxide. The impurities may be removed from the solution either partly or substantially completely.
The term xe2x80x9cbauxitexe2x80x9d as used herein is intended to cover all alumina-containing ores from which alumina trihydrate (e.g., gibbsite) may be obtained.
Anionic impurities present in the solution, which may be removed in the process of the invention, typically include carbonate, sulphate, oxalate (derived from organics in or associated with the bauxite ore), humates, phosphates, silicates and transition metal oxoanions and it is preferably, but not necessarily exclusively, one or more of these species which are removed from the solution, either partially or completely, by intercalation into the double hydroxide. The process of the invention is particularly effective in removing carbonate and/or oxalate ions from the solution. Specific advantages of employing the direct removal of oxalate from Bayer liquors include the fact that the concentration of sodium oxalate in solution in any part of the Bayer plant can be kept below the equilibrium solubility value, and hence any and all unwanted precipitations of sodium oxalate can be prevented.
The present invention can also remove the anionic organic compounds that act as oxalate seed poisons, thereby improving the rate and yield of sodium oxalate precipitation from Bayer liquors, and also removing the danger inherent in operating a Bayer plant in an unstable region with respect to sodium oxalate precipitation, when the crystallization driving force is delicately balanced, and slight changes in the composition of the organic compounds in the Bayer liquor (due to, for example, a change in source of bauxite) can lead to unexpected and unwanted precipitation of sodium oxalate.
The present invention has advantages over the use of other agents for removing impurities from Bayer liquor because of the selectivity of the layered double hydroxides. It can leave in solution aluminate ions as well as the beneficial additives, such as cationic organic species and high molecular weight flocculant molecules. Thus, the additional costs associated with the non-selective removal agents, such as activated carbon, on account of their tendency to remove additives, can be avoided. Furthermore, the ordered arrangement of cations in layered double hydroxides such as those containing layers of formula LiAl2(OH)6+ provides selectivity for different anions and this can be advantageous since it may be possible to select for removal from the liquor only those anions which have a deleterious effect on the process.
The invention is at least partly based on the recognition that some anionic impurities may slow the rate of precipitation of the alumina trihydrate by intercalating into the layers in the trihydrate and/or adsorbing on crystal growth surfaces and blocking active growth sites as the trihydrate crystallises out of the solution. Furthermore, anionic impurities can exhibit other undesirable effects; for example, sulphate ions can act as templates for the crystallisation of zeolites which coprecipitate with the trihydrate and cause problems downstream in the aluminium production process. Oxalate ions play various roles in deleteriously affecting the process and may be considered to constitute the most important impurity. Other impurities present in the solution, such as organic molecules and ions (e.g. humates) may also be removed in the process of the invention.
The process of the invention is preferably based on the conventional Bayer process. The process of the invention is also able to reduce the colour of the conventional Bayer liquor, as formed, by removal of impurities. The layered double hydroxide may be used to treat the Bayer liquor before gibbsite precipitation or after gibbsite precipitation (i.e., by treatment of the green (pregnant) and/or spent liquors) or at both stages of the process. Treatment of the spent liquor reduces the impurities which are recycled with the liquor to the start of the process. Treatment of the spent liquor may be carried out in addition to a conventional step of precipitating oxalate from the liquor.
The process of the invention is applicable in areas other than the Bayer process. Therefore, in another embodiment, the present invention provides a method of producing an aluminium oxide or hydroxide which comprises the precipitation of the oxide or hydroxide from an alkaline solution containing aluminate ions, wherein the method comprises the step of treating the solution with a layered double hydroxide in order to remove impurities from the solution by intercalation into the layers in the double hydroxide. The aluminium oxide or hydroxide produced in this way may be of high purity for speciality use.
The process of the invention is particularly effective in decolourising Bayer liquor to less coloured or colourless solutions. Therefore, another embodiment of the invention is a process for decolourising alkaline solutions containing aluminate anions in the Bayer process which comprises treating the solution with a layered double hydroxide. The decolourisation may be due to the removal of coloured organic anions and/or transition metal oxoanions from the solution.
The layered double hydroxide may be any compound which is able to remove some, or all, of the anionic impurities from the solution. The layered double hydroxide preferably contains aluminium as one of the metal cations in the compound and the other metal cation is preferably ordered within the structure of the compound. A preferred compound comprises layers of formula [LiAl2(OH)6]+, such as [LiAl2(OH)6]OH, optionally hydrated. [LiAl2(OH)6]OH may either be used as such or as a compound of formula [LiAl2(OH)6]A, optionally hydrated, wherein A is a monovalent counterion other than OH such as fluoro, chloro, bromo or nitro, which is converted to [LiAl2(OH)6]OH in situ in the alkaline solution containing the dissolved alumina. However, the use of [LiAl2(OH)6]OH is preferred since it releases only hydroxide ions into the liquor when an impurity is intercalated into the layers in the compound. Thus, [LiAl2(OH)6]OH can act to recausticise the liquor by removing anions such as carbonate and replacing them by hydroxide. Compounds containing [LiAl2(OH)6]+ layers are particularly advantageous since they can be readily produced from the alumina trihydrate (gibbsite) which is precipitated in the Bayer process and one of the starting materials for its production is therefore already available to bauxite processors in large quantities. Also, the use of this alumina-based compound ensures that any leaching of the compound has little or no contaminating effect on the solution. The [LiAl2(OH)6]+ layered compounds are stable at high temperature (above 300xc2x0 C.) and, as mentioned above, can exhibit selectivity for certain anions.
The double hydroxide may be regenerated at the end of the process. A first preferred method of regeneration comprises treatment with carbonate to form a carbonate intercalate, thereby displacing the anionic impurities other than carbonate from the layers in the structure, calcining the carbonate intercalate and hydrating the resulting product. Treatment with carbonate in this regeneration process may involve subjecting the double layered hydroxide to an aqueous solution of a soluble carbonate salt, such as sodium carbonate, at about or above room temperature for up to several hours (e.g., at 20 to 100xc2x0 C. for from 1 to 48 hours). A second preferred method of regeneration involves treatment of the layered double hydroxide, optionally after displacement of the anionic impurities other than carbonate with carbonate (as described for the first preferred method), with an acid under conditions which cause protonation and de-intercalation of the intercalated ions whilst leaving the layers of the layered double hydroxide substantially intact. Suitable conditions involve treatment of the layered double hydroxide with dilute acid (e.g., dilute hydrochloric acid) at room temperature. Regeneration can also be carried out by other methods, however, such as treatment of the double hydroxide with LiCl solution when [LiAl2(OH)6]+ compounds are employed.
In accordance with the conventional conditions of the Bayer process, the solution is preferably treated with the double hydroxide when the solution is at an elevated temperature (e.g., of about 80 to 100xc2x0 C.), preferably by treatment of the spent liquor i.e., after the liquor has been seeded with alumina trihydrate crystals for precipitation. The double hydroxide may be used as such or on a support (e.g., of resin beads). Treatment of the solution with the double hydroxide may involve any method of bringing the two materials together without harmful contamination of the solution such as, for example, passing the solution through the double hydroxide using techniques well-known in filtration and separation science. For example, treatment could take the form of a batch reaction or could involve flow of the liquor through a bed of the layered double hydroxide.
Passing the solution through a body of the double hydroxide allows use to be made of the different intercalation properties of double hydroxides having different guest anions. Thus, for example, it is possible to employ a body comprising a region of formula [LiAl2(OH)6]OH and downstream of said region (i.e., in the direction of flow of the solution), at least one other region of formula [LiAl2(OH)6]Y, wherein Y is an anion other than OHxe2x88x92 whose presence in the solution is beneficial to the process, such that Yxe2x88x92 ions pass into the solution on intercalation of the impurities into the layers in the double hydroxide. In this way, beneficial anions can be added to the solution in precise amounts.
Similarly, it is possible to use, for example, a body of layered double hydroxide comprising a region of formula [LiAl2(OH)6]OH and, downstream of said region, at least one other region of formula [LiAl2(OH)6]Yxe2x80x2, wherein Yxe2x80x2 is an anion which intercalates less strongly into the [LiAl2(OH)6]+ layers than OHxe2x88x92, in order that the region of formula [LiAl2(OH)6]Yxe2x80x2 may intercalate any impurities which have not already been intercalated into the region of formula [LiAl2(OH)6]OH. Y and Yxe2x80x2 may be the same or different.
In another embodiment, the invention provides the use of a layered double hydroxide, such as [LiAl2(OH)6]OH optionally hydrated for example, in the removal of anionic impurities from Bayer process liquors.
The advantages of the present invention include the following:
the layered double hydroxides are readily able to withstand the high pH of the alkaline solutions used to dissolve the alumina;
the double hydroxides may be regenerated;
the double hydroxides can be readily produced from a material which is already available to bauxite processors in large quantities;
the double hydroxides can selectively remove unwanted impurities;
the double hydroxides can recausticise the liquor by exchanging hydroxide ions (OHxe2x88x92) for other ions in the liquor on intercalation;
the intercalation process is very fast at room temperature and above and so large volumes of solution can be treated quickly;
the need for lime or magnesia is avoided and thus so is the detrimental affect of calcium on the precipitation of gibbsite;
the organic and inorganic compounds responsible for the colour of Bayer liquor can be effectively removed;
separation of the unwanted impurities from the solution is obtained quickly without any substantial effect on the aluminate ions contained in the solution and without requiring any significant modification of the conventional Bayer process;
alumina (e.g., gibbsite) growth rates, yield, agglomeration behaviour, morphology, particle size distribution, purity and strength can be improved; and
the use of compounds based on Al(OH)3, such as [LiAl2(OH)6]OH.H2O ensures that the double hydroxide cannot contaminate the solution.